Mitochondrial Nucleic Acids

MDNA is a circular molecule in higher animal cells, but is linear in several eukaryotic cells and plants. MDNA of the L- cell mitochondrion is a circular molecule folded upon itself to give a filamentous branched appearance. MDNA is located in the mitochondrial matrix and may be attached to the membrane.

The MDNA – containing regions of the mitochondrion are called nucleoids. Mitochondria usually have 2 or 3, but sometimes up to 6 nucleoids. In mitochondria ruptured by the osmotic shock method, up to 6 MDNA molecules have been found. In addition to normal MDNA monomers double sized circular molecules or dimmers have also been found. In addition to normal MDNA monomers double sized circular molecules or dimmers have also been found. The dimmer may be attached to another dimmer or monomer by a knob-like central point.

General properties of MDNA. MDNA differs from nuclear DNA in several aspects, and is a distinct species of DNA.

1. MDNA from a wide variety of species shows great similarity, in contrast to nuclear DNA.

2. The melting temperature (TM) of MDNA differs from that of nuclear DNA. Moreover, MDNA, has a rapid rte of renaturation in contrast to nuclear DNA, which recovers slowly from thermal denaturation. The rapid rate of renaturation of MDNA indicates that it has a more homogenous base composition than nuclear DNA.

3. MDNA is usually circular, as in bacteria. The MDNA rings may be open or twisted. The number of rings per mitochondrion varies from 2 to 6.

4. The molecular weight of MDNA varies from 9-11 millions. The much higher MDNA values found in some mitochondria are probably because of the presence of several rings, rather than larger molecules of MDNA. Plants have a high MDNA contents.

5. The base composition of MDNA from different species appears to be rather homogenous. The buoyant densities of most species of MDNA are mostly similar to that of corresponding nuclear DNA.
 
The biosynthesis of MDNA. It has been shown that isolated mitochondria can synthesize DNA. The synthesis of MDNA is to some degree independent of the synthesis of nuclear DNA. MDNA synthesis is not exactly in phase with synthesis of nuclear DNA. Replication of MDNA is semi-conservative in Neurospora.

Function of MDNA. MDNA contains the genetic information for the synthesis of the mitochondrion. However, the synthesis of the entire mitochondrion is not coded by MDNA. Certain mitochondrial proteins are synthesized under the control of nuclear DNA.